In machine tools or the like, there are cases where a motor-driven member cannot be accelerated or decelerated using a single motor because the driven member is large. There are also cases where the driven member cannot be moved in a stable condition because the backlash between the driven member and the machine element of the transmission mechanism connected to the motor drive shaft is large. In such cases, tandem control is performed in which two motors are used to drive a single driven member (for example, refer to Japanese Unexamined Patent Publications No. JP-A-8-174481, JP-A-2003-79180, or JP-A-2004-92859).
In such tandem control, one of the two motors functions as a main motor having a drive shaft that acts as a master shaft connected to the transmission mechanism. The other one functions as a sub motor having a drive shaft that acts as a slave shaft connected to the transmission mechanism.
There is also proposed a motor drive apparatus that generates a preload torque value when performing tandem control in order to suppress the backlash between the driven member and the machine element of the transmission mechanism connected to the motor drive shaft (for example, refer to Japanese Unexamined Patent Publications No. JP-A-8-16246 or JP-A-2010-172054). The preload torque value is a torque value that is appended in advance to the torque command value so that the direction of force applied to the drive shaft of the main motor and the direction of force applied to the drive shaft of the sub motor will become opposite to each other.
In the motor drive apparatus that generates the preload torque value according to the prior art, the magnitude of the preload torque value is constant, irrespective of the magnitude of acceleration of the main motor or sub motor. As a result, during acceleration/deceleration of the main motor, the acceleration/deceleration torque required on the drive shaft of the main motor may become larger than the preload torque value of the main motor. On the other hand, during acceleration/deceleration of the sub motor, the acceleration/deceleration torque required on the drive shaft of the sub motor may become larger than the preload torque value of the sub motor.
In such cases, either the direction of force applied to the drive shaft of the main motor or the direction of force applied to the drive shaft of the sub motor becomes opposite to the direction of the preload torque applied to suppress the backlash. This can result in an inability to suppress the backlash occurring between the driven member and the machine element of the transmission mechanism connected to the motor drive shaft.
It is an object of the present invention to provide a motor control apparatus which, when driving a single driven member using two motors, can suppress the backlash occurring between the driven member and the machine element of the transmission mechanism connected to the motor drive shaft, even during acceleration/deceleration of the motors.